An inventory and estimate of water stored in firn fields, glaciers, debris-covered glaciers, and rock glaciers in the Aconcagua River Basin, Chile

    An inventory and estimate of water stored in firn fields, glaciers, debriscovered glaciers, and rock glaciers in the Aconcagua River Basin, Chile Jason R. Janke, Sam Ng, Antonio Bellisario PII: DOI: Reference:

S0169-555X(17)30376-8 doi:10.1016/j.geomorph.2017.09.002 GEOMOR 6147

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

10 April 2017 31 August 2017 1 September 2017

Please cite this article as: Janke, Jason R., Ng, Sam, Bellisario, Antonio, An inventory and estimate of water stored in firn fields, glaciers, debris-covered glaciers, and rock glaciers in the Aconcagua River Basin, Chile, Geomorphology (2017), doi:10.1016/j.geomorph.2017.09.002

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ACCEPTED MANUSCRIPT An inventory and estimate of water stored in firn fields, glaciers, debris-covered glaciers, and rock glaciers in the Aconcagua River Basin, Chile

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Jason R. Janke*, Sam Ng1, and Antonio Bellisario2

Metropolitan State University of Denver, Department of Earth and Atmospheric Sciences, Denver, CO, 80217,

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USA

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*Corresponding author. Tel.: +1 303-615-0630; E-mail: [email protected] Tel.: +1 303-556-8399; E-mail: [email protected]

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Tel.: +1 303-352-4278; E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract An inventory of firn fields, glaciers, debris-covered glaciers, and rock glaciers was conducted in the Aconcagua River Basin of the semiarid Andes of central Chile. A total of 916 landforms were identified, of which rock glaciers were the most abundant (669) and occupied the most total area. Glaciers and debris-

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covered glaciers were less numerous, but were about five times larger in comparison. The total area

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occupied by glaciers and debris-covered glaciers was roughly equivalent to the total area of rock glaciers. Debris-covered glaciers and rock glaciers were subcategorized into six ice-content classes based on

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interpretation of surface morphology with high-resolution satellite imagery. Over 50% of rock glaciers fell within a transitional stage; 85% of debris-covered glaciers were either fully covered or buried. Most

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landforms occupied elevations between 3500 and 4500 m. Glaciers and firn occurred at higher elevations compared to rock glaciers and debris-covered glaciers. Rock glaciers had a greater frequency in the

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northern part of the study area where arid climate conditions exist. Firn and glaciers were oriented south, debris-covered glaciers west, and rock glaciers southwest. An analysis of water contribution of each

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landform in the upper Andes of the Aconcagua River Basin was conducted using formulas that associate the

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size of the landforms to estimates of water stored. Minimum and maximum water storage was calculated based on a range of debris to ice content ratios for debris-covered glaciers and rock glaciers. In the

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Aconcagua River Basin, rock glaciers accounted for 48 to 64% of the water stored within the landforms analyzed; glaciers accounted for 15 to 25%; debris-covered glaciers were estimated at 15 to 19%; firn fields contained only about 5 to 8% of the water stored. Expansion of agriculture, prolonged drought, and removal of ice-rich landforms for mining have put additional pressure on already scarce water resources. To develop long-term, sustainable solutions, the importance of the water stored in rock glaciers or other alpine permafrost landforms, such as talus slopes, must be weighed against the economic value of mineral resources.

Keywords: Glaciers; Debris-Covered Glaciers; Rock Glaciers; Water Resources

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ACCEPTED MANUSCRIPT 1. Introduction 1.1. Background Water resources are vulnerable to climate change in many arid mountain regions of the world (Rangecroft et al., 2013, 2016). Chile has abundant water resources, but the distribution is highly uneven

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over the large latitudinal range of the country (28.5˚ total); arid conditions in the north transition to wet

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conditions in the south. Unfortunately, the northern part of the country contains most of the population as well as water-dependent economic activities, such as agriculture, mining, and industry (Universidad de

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Chile, 2010). Expansion of mining and agriculture, when combined with a growing population, prolonged droughts, and glacial recession, has placed a growing pressure on the availability of water resources

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(Valdés-Pineda et al., 2014).

Rock glaciers contain interstitial ice or an ice-core that has been consolidated with moraine material

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or talus delivered through rockfall; debris-covered glaciers have a greater ice content as well as a layer of supraglacial debris that has been delivered by avalanching or rockfall (Table 1) (Berthling, 2011; Perucca

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and Angillieri, 2011). Despite having different internal ice structures and varying ice contents, rock glaciers

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are often interpreted as debris-covered glaciers (Barsch and King, 1975). Genetic and morphological classification schemes have been used to describe and classify glaciers in the Andes (Corte, 1976a, b; Soto

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et al., 2002; Ferrando, 2003, 2012; Brenning, 2005; Ferrando et al., 2003; Geoestudios, 2008a,b; Nicholson et al., 2009; Bodin et al., 2010; Brenning and Azócar, 2010; Berthling, 2011). A classification system proposed by Janke et al. (2015) defined glacier categories based on the ice content to better estimate available water resources to assist planners (Table 1). Debris-covered glaciers are categorized into semi-, fully, and buried categories, whereas rock glaciers are considered to exist in proper, transitional, and glacier of rock forms (Table 1).

Landforms, such as rock glaciers and debris-covered glaciers, that are less sensitive to climate change because of an insulating, productive surface cover are expected to serve as a significant water source in the future. Until recently, the water stored in these landforms has been unrecognized. Assessing available water resources is important so that informed decisions of future water supply can be made. Thus, some initial inventories of landforms of the cryosphere have been undertaken in the Andes of South America. In the Monte San Lorenzo region in the southern Andes of Patagonian Argentina, 130 rock glaciers were identified (Falaschi et al., 2015) (Fig. 1). A total of 94 rock glaciers were identified in the Bolivian Andes using Google Earth ©, 57% of which were active (Rangecroft et al., 2014). In the Valles 3

ACCEPTED MANUSCRIPT Calchaquíes area near Salta in the northwestern region of Argentina, the Advanced Land Observing Satellite PRISM (Panchromatic Remote-sensing Instrument for Stereo Mapping) with a resolution of 2.5 m was used to identify 488 rock glaciers covering an area of 59 km2 (Falaschi et al., 2014). The Argentinian Cordillera Oriental contains small but numerous (635) rock glaciers, of which 25% are active (Martini et al., 2013). In

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the San Juan Frontal Cordillera, Argentina, 155 rock glaciers were identified, of which 85 were active (Angillieri, 2009, 2010; Perucca and Angillieri, 2011). In the San Juan and Mendoza provinces of northwest

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Argentina, 4200 glaciers were inventoried with a surface area of 1564 km2, about 50% of the total were

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rock glaciers (Bottero, 2002) (Fig. 1).

In northern Chile, a few recent detailed glaciological studies developed a baseline of cryosphere

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landforms. In the Andean catchment of the Huasco River Basin (29° S), ASTER images (15 m) and aerial photos were used to identify 152 landforms covering an area of 23.2 km 2, of which 40 were active rock

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glaciers covering 6.3 km2 (Nicholson et al., 2009). In a different study, 9.4 km2 of glaciers, 0.95 km2 of debris-covered glaciers, and 22.5 km2 of rock glaciers were mapped in the Huasco River Basin (Medina et al.,

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2015). In an area of the Andes, along the arid diagonal (25°17´ to 27°42’ S) near the Atacama Desert, 5.9

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km2 of rock glaciers were identified using Google Earth © and Landsat 8 OLI/TIRS imagery (Amigo et al., 2015). For the Copiapó River Andean catchment (27° S), 16.9 km2 of glaciers, 1.1 km2 of debris-covered

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glaciers, and 15.9 km2 of rock glaciers were identified (Ulloa et al., 2015) (Fig. 1).

1.2. Previous glacier inventories in the Aconcagua River Basin Despite the proximity to the largest urban centers of the country, the glaciology of the Aconcagua Basin has been understudied (Table 2). The first glacier inventory in the Aconcagua Basin was conducted in the early 1980s using 1955/1956 aerial photographs in which 267 glaciers were identified and that covered a glaciated area of 151.3 km2 (Valdivia, 1984). Most landforms identified were glaciers with exposed ice. The average elevation of glaciers was 3702 (± 183 m). Most of the glaciers had a southwest, south, or southeast orientation. The total ice volume was estimated at 7.1 km3 with a water equivalent of 5.7 km3 (based on an ice density of 0.8 g/cm3) (Valdivia, 1984). The total glaciated area in the Aconcagua River Basin has decreased by 30 km2 from 1955 to 2003—a 20% reduction (Bown et al., 2008). A total of 159 glaciers were identified; however, partial debris-covered glaciers were classified as glaciers, and rock glaciers were not inventoried. The Juncal Norte Glacier (7.6 km2) and the Glacier del Río Blanco (24.3 km2) were the largest inventoried glaciers, located in the southern end of the basin (Bown et al., 2008). 4

ACCEPTED MANUSCRIPT In 2008, the Dirección General de Aguas (DGA) commissioned a study to identify rock glaciers in the Andes of central Chile using updated aerial photos and improved cartographic methods. A total of 628 rock glaciers were identified in the Aconcagua River Basin; however, surface area was not measured (Geoestudios, 2008b). Another study was also commissioned by the DGA in 2008 in which a total of 188

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glaciers with an area of 121.5 km2 were identified in the Aconcagua River Basin with ASTER satellite images (15 m) from 2003 and 2008. Rock glaciers occupied the greatest area (61.6 km2), followed by glaciers (52.6

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km2) and debris-covered glaciers (7.3 km2) (Centro de Estudios Científicos, 2008). Given the coarse

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resolution of the ASTER imagery, rock glaciers could not be completely identified. In 2011, the DGA commissioned an inventory of only rock glaciers in the Aconcagua River Basin

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using a combination of aerial photography, satellite imagery, digital cartography, and geotechnologies (Geoestudios, 2011). This new study identified 519 rock glaciers, which covered 86.5 km2 with an

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estimated volume of 2958 km3. The average elevation of rock glaciers was 3762 m (maximum 3894; minimum 3665 m). The predominant aspect of rock glaciers was south, southeast, and southwest. Most

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rock glaciers were in the subbasins of Río Blanco and Río Colorado (Geoestudios, 2011).

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These later studies clearly demonstrate the importance of glaciers and rock glaciers in the Aconcagua River Basin. Rock glaciers outnumbered glaciers and debris-covered glaciers and the total area

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covered by rock glaciers was more than half of the total glaciated area in basin (Table 2). These studies, however, have not provided a single, complete inventory of the entire basin. In addition, they have used different classification schemes and methods and have reported different results. Many of these studies do not differentiate among glacial forms, in particular debris-covered glaciers and rock glacier subclasses, which have different geographic characteristics (location and area), topographic variables (elevation, slope, and aspect), and water storage capabilities. This study provides a comprehensive inventory of landforms in the Aconcagua River Basin, utilizing a novel classification based on estimated ice volume to determine water content (Janke et al., 2015).

1.3. Study area The largest share of the population of Chile and the most important economic activities (mining, agriculture, and industry) are concentrated in the northern half of the country (from the northern/upper boundary with Perú 17°30' S to the capital city of Santiago 34° S). This region is experiencing water scarcity because of its arid climate, higher water demands, and prolonged droughts (Programa Chile Sustentable, 5

ACCEPTED MANUSCRIPT 2004). This northern region also has the largest population and a large percentage of its economic activity; both are highly dependent on water supplied by melting snow and ice originating in the high altitude of the Andes cordillera (Salazar, 2003; Universidad de Chile, 2010). In basins with more than 20% of their area covered by glaciers, the glacial contribution to streamflow could reach 100% during dry years, whereas in

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less substantially glaciated areas (<6%), contribution could reach about 57% (Baraer et al., 2012; Ragettli and Pellicciotti, 2012; Ohlanders et al., 2013).

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The Aconcagua River Basin, with a total area of 7340 km2, is the second most productive irrigated

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valley in Chile after the Maipo River valley (Figs. 1, 2). The length of the valley is about 130 km and the width is roughly 100 km. The landscape in the basin contains mountainous terrain with steep, narrow river

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valleys. Elevation in the basin ranges from sea level to 6100 m. The Aconcagua River provides water for agriculture, mining, and industry and freshwater for human consumption for the 1.7 million inhabitants of

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the Valparaíso region, which represents about 9.6% of the total population of Chile (Dirección General de Aguas, 2002, 2012). Historically, snowline has occurred at 2500 m and has since risen from about 2500 m

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in 2001 to about 3000 m in 2015 (Center for Climate and Resilience Research, 2015). The most important

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urban centers are the Valparaíso-Viña del Mar conurbation with 1 million inhabitants (the second largest metropolitan region in Chile) and Los Andes and San Felipe with 160,000 and 71,000 inhabitants

1.4.

Objectives

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respectively (Figs. 1, 2) (Dirección General de Aguas, 2004a, b).

The objectives of this paper are threefold: (i) update and reclassify the existing inventory of firn, glaciers, debris-covered glaciers, and rock glaciers using fine resolution (0.5 m) satellite imagery in the Aconcagua River Basin; (ii) examine the geographic distribution and determine the topographic characteristics (elevation, slope, aspect) of each landform; and (iii) estimate potential water storage provided by these landforms in the Aconcagua River Basin.

2. Methods 2.1. Digitizing, classification, and data extraction An inventory of landforms was obtained from the Dirección General de Aguas (DGA). Polygons were constructed using a combination of sources: ASTER imagery (15 m) from 2009, historical aerial photography, digital air photos from 2006, SPOT-5 imagery (2.5/5 m) from 2011, and Rapideye imagery (5 m) from 2011. The resolution of the imagery was often coarse, which introduced uncertainty into the classification. In 6

ACCEPTED MANUSCRIPT addition, inconsistent attributes were assigned to features. To correct and update, the existing inventory was overlaid on a composite of Digital Globe imagery (0.5 m) from January 2010. Using ArcGIS 10.4 ©, additional features, which were missing from the original inventory because they were not identifiable, were added. Features were edited and redigitized to accurately depict the extent of the landforms.

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After digitizing, the landforms were classified based on surface characteristics (Table 1) (Janke et al.

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2015; Monnier and Kinnard, 2015a). Polygons were overlaid on elevation, slope, and aspect grids (30 m resolution), and a zonal function was used to generate summary statistics (minimum, maximum, mean, and

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standard deviation) for each class of landform. For zonal statistics, a polygon was used to define an area of interest on a raster grid; cells within the polygon were summed and statistics were calculated. Circular

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aspect data were converted to vector space to determine mean exposure.

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2.2. Estimates of water contained

Formulas were developed based on ice-coring data to estimate water resources stored in each class

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of landform (Croce and Milana, 2002; Brenning, 2005; Geoestudios, 2008a, b; Monnier and Kinnard, 2013,

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2015b, c; Monnier et al., 2014; Rangecroft et al., 2015). Area of each landform was calculated in ArcGIS 10.4 ©. The thickness of each feature was estimated using a formula developed from coring field data that

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relates area to thickness using a best of fit power function regression curve (Brenning, 2005; Geoestudios, 2008a, b, 2011) (Table 1). Ice content was estimated based on a range of values to account for the variability within debris-covered glaciers and rock glaciers as well as potential uncertainty associated with the developed formula (Table 1). Estimates of water stored were calculated by multiplying by the density of ice (0.9 g/cm3) and firn (0.7 g/cm3) for respective landforms.

3. Results 3.1. Landforms and statistics A total of 916 landforms of the cryosphere were identified, covering a total area of 144.9 km 2. Rock glaciers were the most abundant (669), which accounted for 73% of the total (Table 3; Fig. 3). Because of their high frequency of occurrence, the total area covered by rock glaciers was the greatest (70.0 km 2) compared to other landforms (Table 3). Firn fields (128) were the second most abundant, but were smaller in size, thus covering the smallest total area (4.9 km2) (Table 3). Glaciers (61) and debris-covered glaciers (58) were less abundant, but were about five times larger than rock glaciers on average. Historically, the winter snowline existed at about 2000 m in this area; however, it has risen from about 2500 m in 2001 to 7

ACCEPTED MANUSCRIPT 3000 m in 2015 (Center for Climate and Resilience Research, 2015). These elevations represent the lower limit of the watershed that can receive meltwater from the landforms containing ice. Rock glaciers occupy the greatest area above the historical snowline. In terms of area, their importance increases as the snowline rises to 3000 m (Table 3).

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About 15% of all debris-covered glaciers fell within class 1 (semicovered), 42% were class 2 (fully

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covered), and 43% were class 3 (buried). Compared to the other classes, Class 3 (buried) was the most abundant and covered the most area. When examining rock glaciers, class 5 (transitional) was the most

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abundant, but class 4 (proper) covered the largest area (33.3 km2). About 24% of all rock glaciers were class 4 (proper), 54% were class 5 (transitional), and 22% were class 6 (glaciers of rock).

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3.2. Topographic characteristics

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Debris-covered glaciers and rock glaciers occupy lower elevations compared to the other landforms

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examined (Table 4). This is likely the result of protective debris cover, which insulates ice and allows them to exist at lower elevations, ~700 m lower than uncovered glaciers (Table 4). Glaciers are located at the

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highest elevation (4413 m) but have the most variability (± 477 m), which is the result of verticality. Firn was located about 250 m lower than glaciers on average (Table 4). Classes 4 (proper) and 5 (transitional) rock glaciers had similar mean elevations (about 3810 m); class 6 (glacier of rock) was about 300 m lower on average than the aforementioned classes. The similar elevations between classes 4 and 5 indicate that topographic variables that influence snow distribution and cover, solar radiation received (aspect, slope, shading), or available debris from moraines or talus slopes are important for maintaining ice content. Class 2 (fully covered) had the highest mean elevation (3920 m); class 3 (buried) had intermediate mean elevation (3763 m); and class 1 (semicovered) had the lowest mean elevation (3592 m). Semicovered glaciers may not be adjusted to the current climate given the low elevation and may continue to retreat to higher elevations as has been the case with the debris-covered Juncal glacier, which has retreated at a rate of 14 m/y (Rivera et al., 2002; Nicholson et al., 2009; Centro de Estudios Científicos, 2011). Most landforms examined occur between 3500 and 4500 m (Fig. 4). Rock glaciers occur with a greater frequency in the north because of arid conditions (Fig. 4). Debris-covered glaciers extend across a range of latitudes. Glaciers and firn fields increase in abundance toward the south because of greater snowfall (Fig. 4). Mean slope was similar for rock glaciers (20°) and debris-covered glaciers (17˚), whereas 8

ACCEPTED MANUSCRIPT glaciers and firn occupied steeper slopes (about 27˚) and had greater variability (± 12˚) because of the location at higher elevations that exhibit extreme verticality (Table 5). Class 3 (buried) debris-covered glaciers had the least slope (15°) compared to the other debris-covered glacier classes, which may be the



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result of flattening from ice loss. Rock glaciers subclasses were all close to 20° (Table 5).



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Mean aspect was calculated for each of the landform classes (Table 6). Firn and glaciers had a mean southerly orientation. Debris-covered glaciers faced west; rock glaciers faced southwest. The orientation

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of the valleys appears to be correlated with the orientation of the landforms (Fig. 5). A rock glacier belt occurs from 3500 to 4250 m at a range of aspects; however, rock glaciers occur with less frequency to the

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northwest, which again may be the result of the orientation of the valleys in the Andes (Fig. 5).

3.3. Water resources

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The amount of water stored in the landforms was estimated using formulas that relate thickness to area and ice content (Tables 1, 7). A range of values is provided for rock glaciers and debris-covered glaciers to account for variability and uncertainty of estimates of rock to ice ratios. The estimate of total water stored in the cryosphere landforms analyzed ranges from 4595 to 7540 km 3. Of the total water stored in the examined landforms, firn consists of 5 to 8% (373 km 3) of the total; glaciers have 15 to 25% (1145 km3); debris-covered glaciers have 15 to 19% (878 to 1165 km3); and rock glaciers have 48 to 64% (2200 to 4858 km3). Despite having a greater ice content, the area covered by class 1 (semi-) debriscovered glaciers is smaller, which is a less significant source of water storage compared to other debriscovered classes. Most of the water stored is in class 4 (proper) and 5 (transitional) rock glaciers, which cover a larger area and have a greater ice content compared to class 6 (glacier of rock). Water stored as ice was unevenly distributed within the study area. The Primera section (south) has 6 to 10 times more water stored compared to the Putaendo (north) (Fig. 2; Table 8). In the Primera, the greatest potential contribution comes from rock glaciers. In the Putaendo, rock glaciers stored the greatest amount of water; however, this is about four times less than what is stored in the Primera. Because the 9

ACCEPTED MANUSCRIPT snowline has risen by about 1000 m, the receiving area of the catchment has been reduced by about 50%, increasing the importance of melting cyrosphere landforms to streamflow. Rock glaciers provide a significant contribution to each subcatchment (Table 9). The Río Blanco and the Río Juncal have the



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greatest glacial contribution because large glaciers exist in these areas (Table 9).

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4. Discussion 4.1. Characteristics and distribution of landforms

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Compared to previous inventories, a greater number (~300) of total landforms were identified. These were previously unidentifiable because of new satellite image sources (Tables 2, 3). The area of

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glaciers characterized in this study was less than what had previously been inventoried, which may suggest a reduction in area or transition to other forms (Table 2; Table 3). For instance, Valdivia (1984) identified

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151.0 km2 of glaciers, whereas only 39.9 km2 of glaciers were classified in this study. According to previous

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studies, rock glaciers covered from 61.6 to 86.5 km2 and debris-covered glaciers covered 7.3 km2. In this study, 70.0 km2 of rock glaciers and 30.2 km2 of debris-covered glaciers were identified. Differences may be

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the result of image sources or of the classification schemes utilized. Consistently, thoroughly, and accurately identifying landforms is important so that complete inventories can be used in monitoring and water conservation projects.

Mean elevation of all landforms ranged from 3510 to 4413 m, which was similar compared to other sections of the semiarid Andes and to previous inventories conducted in the Aconcagua River Basin (CECS, 2008; Geoestudios, 2011; Perrucca and Angillieri, 2011). Landforms exhibited a similar pole-facing trend that has been observed in other mountain ranges of the world (Table 6; Fig. 5) (Janke, 2007). Firn and glaciers occurred only at extreme southern-facing slopes (~190°); debris-covered glaciers and rock glaciers had greater insulation from protective rock that allows them to exist at a wider range of aspects (Table 6). Rock glaciers extended to lower elevations on southern slopes because of protective debris cover (Fig. 5). Debris-covered glaciers and rock glaciers had similar mean elevations (3787 and 3766 m respectively), slopes (17 and 20°), and aspects (264 and 217°). This illustrates the importance that topoclimates and high talus production or the availability of moraine material have on formation (Janke, 2007). Giardino and Vitek (1988) used a glacial to periglacial continuum to describe transition of rock glaciers. A similar continuum exists in the semiarid Andes via evolution of glaciers to debris-covered 10

ACCEPTED MANUSCRIPT glaciers and eventually to rock glaciers. According to surface morphology, glaciers transition into debriscovered glaciers that are semi-covered (Table 1). As debris cover thickens, debris-covered glaciers become fully covered and then buried. This is supported by the location of the landforms on the hillslope, which suggests a transition as glaciers recede and become other forms. Glaciers are found at higher mean

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elevations (4413 m) on steep (27°) southern-facing slopes. They are often detached from debris-covered

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glaciers (3787 m) or rock glaciers (3766 m) that form beneath them in valleys that are more gently sloping (15 to 20°). A transition to rock glacier form occurs when debris content reaches more than 50%. The

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majority of rock glaciers (364) fell within class 5, a transitional category in which ice, and therefore rates of flow, are reduced until they reach class 6 or glaciers of rock. This decay cycle will continue unless large-

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scale events, such as climate cooling or mountain uplift, alter the continuum. Water stored as ice will

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decline as landforms continue to transition through the cycle.

4.2. Water resources

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Rock glaciers have the most significant water storage potential in the Aconcagua River Basin. Rock

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glaciers comprise roughly 50% of the water storage. Their importance increases where ice glaciers are absent (Rangecroft et al., 2015). To the north, rock glaciers are also significant stores of ice between 29˚

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and 32˚ S (Azocar and Brenning, 2010). Releases of water on the Tapado glacial complex were highly correlated with daily and monthly weather conditions (Pourrier et al., 2014). Water transfer through cryospheric compartments, however, was complex, influencing the timing of meltwater release. Melting internal ice drives late summer supply of water to the Elquí River Basin, which could supply an important source of late summer water for agriculture (Falaschi et al., 2014). A significant source of late summer meltwater originates from rock glaciers in the Aconcagua River Basin. Peak discharge is driven by snowmelt during spring months (November-December) and glacial-melt during the mid- to late-summer months (February-March) (Fig. 6). During extremely dry years (La Ñina), the glaciated area contributes about 50 to 90% of the discharge (Montecino and Aceituno, 2003; Masioka et al., 2006). The melting of stored water in glaciers and permafrost maintains the streamflow of the river during the summer season, when demands for freshwater and for irrigation are the highest. Agricultural dependence on meltwater from the upper catchment is significant in the Aconcagua River Basin because of the absence of major reservoirs to regulate water supply. In 2016, the government completed the construction of the Chacrillas reservoir in the upper Putaendo River to provide a more dependable and regulated supply of water (Venezian, 2013). 11

ACCEPTED MANUSCRIPT Water stored in other periglacial landforms, such as talus slopes, should also be considered when evaluating the availability of water resources. Because of their coarse nature, water stored in talus slopes was thought to be minimal; however, new evidence suggests more storage potential than previously

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thought (Weekes et al., 2015). Spring water has been observed emerging from the base of talus slopes in

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the Sierra Nevada, which was slightly warmer than water emerging through rock glaciers (Millar et al., 2013). Boreholes in the Swiss Alps revealed talus that was partially or slightly supersaturated with ice

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(Scapozza et al., 2015). In the Aconcagua River Basin, talus covers about 65% of the elevation above 2,000 m.

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A formula was developed to calculate the contribution to discharge of different cryosphere landforms inventoried in the Andes of the Atacama Desert (Garcia, 2015; Medina et al., 2015;). Discharge

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for each landform was directly measured in the field and correlated with landform area. Relative hydrologic productivity in the dry Andes was estimated using the total area of each landform in hectares

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multiplied by the following constants: glaciers = 0.64 l/s, debris-covered glaciers = 0.376 l/s, rock glaciers =

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0.2 l/s, and protalus lobes and talus = 0.043 l/s (Milana, 2005). This may underestimate the contribution to streamflow in the semiarid conditions of the Aconcagua River Basin, which has greater precipitation and

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larger glaciers; however, it should provide a general approximation of the contribution to streamflow. Annual average flow was estimated at 14.0 m3/s (Table 10). The area of talus was calculated at 2,075 km2, which corresponds to a discharge of 8.9 m3/s. The annual average discharge of the Aconcagua is 32 m3/s (the maximum is 69 m3/s and the minimum is 11 m3/s). From this total, glaciers with 1.8% of the total area contribute 18% to streamflow, rock glaciers with 3.2% of the total area contribute 10%, debris-covered glaciers with 1.4% of the total area contribute 8%, and talus with 93% of the total area contributes 64% (Table 10).

4.3. Pressure on water resources Central Chile receives nearly all its precipitation during the winter; however, it normally experiences wet and dry cycles, with extreme interannual variability. Anthropogenic climate change is likely to have an impact of extreme ENSO phases which will likely affect water availability in the future (Waylen and Caviedes, 1990; Cai, 2014). Since 2010, total annual precipitation has averaged only 144 mm, whereas the historical average (1941 to 2009) was 307 mm. This represents a 53% decrease in total annual precipitation 12

ACCEPTED MANUSCRIPT (Resguardo Los Patos hydroclimatic station, 32°29’ S, 70°35' W; 1218 m). Precipitation at other stations in the Aconcagua River Basin also have deficits that range from 45 to 83% (Venezian, 2013). During the spring and summer, when water demand is high, ice- and snowmelt in the upper catchments provide most of the streamflow for the river basins (Pellicciotti et al., 2007). Glacier contribution to total runoff is crucial in arid

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and semiarid regions that are prone to droughts and have high interannual precipitation variability. In the

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arid northern zone (18 to 27° S), the cryosphere contributes about 70% to annual river discharge (Milana, 2005). During periods of droughts, permanent ice and permafrost compensate for the deficit, avoiding

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critical water shortages in the Atacama Desert (Amigo et al., 2015; Espinoza et al., 2015; García et al., 2015; Ulloa et al., 2015). For the Maipo River (33°30’ S) to the south, glacial melting contributes up to 67% of the

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total runoff during periods of drought (Peña and Nazarala, 1987).

The Aconcagua is a relatively small river with an average discharge of 32 m3/s. Between 1990 and

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2012, total water demand for consumptive and nonconsumptive uses more than doubled in the Valparaíso Region, from 61 to 152 m3/s. Surface water availability in the Aconcagua Basin, however, is only 801

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m3/inhab/y, which is below the threshold for a basin to be considered under water scarcity (1000

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m3/inhab/y) (World Bank, 2011). By the year 2021, total demand will increase to 188 m3/s, a 23% increase (Venezian, 2013). The main uses of the Aconcagua River are irrigation (more than 10,000 agricultural farms

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in the valley), industry, domestic water usage, and mining. Since the year 2000, the irrigated land in the Aconcagua Basin has increased by at least 15%. The Greater Valparaíso-Viña del Mar, the second largest metropolitan area in the country with about 1 million inhabitants, and the large copper mine, Andina/SurSur located in the upper basin, also utilize water from the Aconcagua River (Dirección de Obras Hidráulicas, 2015).

Agricultural expansion, coupled with the current megadrought since 2000, has put pressure on water resources during the summer (Center for Climate and Resilience Research, 2015). The mean annual 0°C air temperature altitude for central Chile exists at 3600 m and the equilibrium line altitude (ELA) at about 4400 m (Carrasco et al., 2005). By mid-summer, < 10% snow cover typically exists within the basin, suggesting that snowpack is a seasonal, temporary water resource. The water stored within debris-covered glaciers and rock glaciers provide a significant late season addition to streamflow to support irrigation during drought. These landforms, however, are being removed for exploitation of mineral resources, such as copper. To develop long-term, sustainable solutions, the importance of the water stored in rock glaciers or other alpine permafrost landforms (such as talus slopes) must be weighed against the economic value of mineral resources. 13

ACCEPTED MANUSCRIPT 5. Conclusions A total of 916 firn fields, glaciers, debris-covered glaciers, and rock glaciers were mapped. Rock glaciers were the most abundant (669), which accounted for 73% of the total. Glaciers and debris-covered

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glaciers were less numerous, but were about five times larger in comparison. Most landforms occurred

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between 3500 m and 4500 m. The total area occupied by glaciers and debris-covered glaciers was roughly equivalent to the total area of rock glaciers. Glaciers and firn occurred at higher elevations compared to

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rock glaciers and debris-covered glaciers because of a protective surface layer that allows them to exist at lower elevations. Rock glaciers had a greater frequency in the northern part of the study area where arid

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climatic conditions exist. Firn and glaciers were oriented south; debris-covered glaciers west; and rock glaciers southwest. Total water stored in the landforms analyzed was predominantly in rock glaciers: rock

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glaciers (48 to 64%); glaciers (15 to 25%); debris-covered glaciers (15 to 19%); and firn (5 to 8%). The Primera section (south) has 6 to 10 times more water stored in the cryosphere compared to the Putaendo

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(north) with the greatest potential contribution coming from rock glaciers. Water stored in talus slopes is

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likely to be a more significant contributor compared to firn, glaciers, rock glaciers, and debris-covered glaciers because talus covers a large area in the semiarid Andes. As snowline rises, meltwaters from

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periglacial landforms will be essential to maintain flow of the Aconcagua River. The Aconcagua Basin has experienced a rise in mean annual air temperature, a decrease in total precipitation, and increased agricultural water demands. This has led to conflicts among water use for agriculture, mining, and domestic purposes (Bellisario et al., 2014). Water security will continue to be a problem for some of the fastest growing regions in South America.

Acknowledgements We would like to thank Marco Marquez and Francisco A. Ferrando for their logistical support in Chile. Appreciation is expressed to Catherine Kenrick and Tomás Dinges for providing access to conduct fieldwork in the Parque Andino Juncal. Special thanks are offered to Dick Marston (editor) for his patience, dedication, and attention to editorial detail. Three anonymous reviewers, Jack Vitek, and Rick Giardino provided critical feedback to improve this manuscript. Fieldwork was supported by many grants from the Department of Earth and Atmospheric Sciences, the Dean of the College of Letters, Arts and Sciences, the Provost, the Office of International Studies, and the Office of Sponsored Research and Programs at the Metropolitan State University of Denver. 14

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Andes of Argentina and Chile, 1951 2005: Large-Scale Atmospheric Influences and Implications for Water Resources in the Region. J. Clim. 19(24), 6334-6352. http://dx.doi.org/10.1175/JCLI3969.1

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ACCEPTED MANUSCRIPT Waylen, P., Caviedes, C., 1990. Annual and seasonal streamflow fluctuations of precipitation and streamflow in the Aconcagua River basin, Chile. J. Hydro. 120, 79-102. http://dx/doi.org/10.1016/0022-1694(90)90143-L Weekes, A., Torgersen, C., Montgomery, D., Woodward, A., and Bolton, S., 2015. Hydrologic response to

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valley-scale structure in alpine headwaters. Hydrol, Process. 29(3), 356-372.

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ACCEPTED MANUSCRIPT Fig. 1. Locations of previous studies in relation to the Aconcagua River Basin

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Fig. 2. Watersheds and administrative hydrologic units of the Aconcagua River Basin.

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Fig. 3. An example of landforms of the cryosphere identified as part of the inventory.

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Fig. 4. Scatterplot of latitude versus elevation across the Aconcagua River Basin.

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Fig. 5. Polar diagram showing the relationship between the aspect and elevation of analyzed landforms.

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Fig. 6. Monthly precipitation and streamflow for the Aconcagua River (Vilcuya station 1941 to 2009).

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ACCEPTED MANUSCRIPT Table 1 Types, classes, common names, characteristics, and ice content of cryosphere landforms

Table 2

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Summary of glacier inventories of the Aconcagua Basin

Count and size information is provided for landforms

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Table 3

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Elevation data for landforms

Table 5

Table 6

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Slope data obtained for the landforms

Table 7

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Mean aspect data for analyzed landforms

Estimates of water resources for landforms of the cryosphere Table 8

Estimates of water resources for Aconcagua water regions

Table 9 Breakdown of water contribution for each subcatchment

Table 10 Estimates of water contribution to discharge in the Aconcagua River Basin according to landform type

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ACCEPTED MANUSCRIPT Table 1 Common name

Characteristics

Ice content (%)

Class 1

Semi

Debris in the ablation zone (toe)

Class 2

Fully

Fully covered with debris (95%); glacier is disconnected from the head of the covered part

Class 3

Buried

Fully covered with debris (thick > 3m); thermokarst is present

45-64.9

Class 4

Proper

Transverse ridges and furrows are arched - steep front slope differentiated by color

25-44.9

Class 5

Transitional

Transverse ridge and furrows are linear - movement has ceased

10-24.9

Class 6

Glacier of rock

Surface topography is subdued, rounded, weathered away

0.1-9.9

Thickness (m)

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85-100

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Rock Glacier

Class

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Type Debriscovered

Dense ice with flow features and banding

Firn

Ice at an intermediate stage between snow and ice; appears as old snow that has no indication of flow

100

AC

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Glaciers

65-84.9

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ACCEPTED MANUSCRIPT Table 2 Inventory Valdivia (1984)

Type

Count

Glacier

Area 2 (km )

267

151.0

159

121.2

Debris-covered

Glacier Debris-covered

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Bown et al. (2008)

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Rock Glacier

Rock Glacier

Glacier Debris-covered Rock Glacier

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Debris-covered Rock Glacier 628 52.6 7.3

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Centro de Estudios Científicos (2008)

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Glacier Geoestudios (2008a, b)

61.6

86.5

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Debris-covered Rock Glacier 519

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Geoestudios (2011)

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Glacier

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Count

Percentage of area above 2000 m

Percentage of area above 3000 m

Average size (km2)

14%

4.9

0.16%

0.25%

0.04

Glacier

61

7%

39.9

1.29%

2.05%

0.65

Debris-Covered

58

6%

30.2

0.98%

Class 1

9

16%

3.9

0.13%

Class 2

24

41%

9.0

0.29%

Class 3

25

43%

17.3

0.56%

669

73%

70.0

Class 4

161

24%

33.3

Class 5

364

54%

25.6

Class 6

144

22%

11.1 145.0

0.43

0.46%

0.38

0.89%

0.69

2.27%

3.59%

0.10

1.08%

1.71%

0.21

0.83%

1.31%

0.07

0.36%

0.57%

0.08

RI

0.20%

TE

D

MA

916

0.52

AC CE P

Total

1.55%

SC

Rock Glacier

PT

128

NU

Firn

Total Percentage area of total (km2)

32

ACCEPTED MANUSCRIPT Table 4

4842

4161

301

Glacier

3322

5900

4413

477

Debris-Covered

2896

4656

3787

283

Class 1

2896

4656

3592

378

Class 2

3104

4626

3920

262

Class 3

3060

4419

3763

232

2370

4565

3766

294

Class 4

2982

4528

3810

Class 5

3187

4565

3819

Class 6

2370

4393

3510

391

MA

243 244

AC CE P

TE

D

Rock Glacier

RI

3196

SC

Firn

NU

Type

PT

Mean Standard Minimum Maximum elevation deviation (m) (m) (m) (m)

33

ACCEPTED MANUSCRIPT Table 5 Minimum slope (°)

Type

Maximum slope (°)

Mean slope (°)

Standard deviation slope (°)

0.8

70.1

26.8

12.1

Glacier

0.0

75.9

27.2

13.3

Debris-covered

0.0

64.9

17.1

9.4

Class 1

0.4

63.9

20.0

10.7

Class 2

0.4

64.8

19.2

Class 3

0.0

64.9

15.4

0.0

68.5

20.3

Class 4

0.0

68.5

19.6

Class 5

0.0

67.4

Class 6

0.4

57.2

RI 9.7

NU

SC

8.5 9.0 9.0 9.2

19.6

8.4

TE

D

MA

21.6

AC CE P

Rock Glacier

PT

Firn

34

ACCEPTED MANUSCRIPT Table 6

S

Debris-Covered

264

W

Class 1

336

NW

Class 2

275

W

Class 3

218

SW

217

SW

Class 4

228

SW

Class 5

215

SW

Class 6

195

S

D TE AC CE P

Rock Glacier

PT

190

MA

Glacier

RI

Firn

Direction S

SC

Type

NU

Mean aspect (°) 196

35

ACCEPTED MANUSCRIPT

372.8

8%

372.8

5%

1144.5

25%

1144.5

15%

877.7

19%

1164.7

15%

Class 1

190.3

4%

223.9

3%

Class 2

371.4

8%

485.1

6%

Class 3

316.0

7%

455.7

6%

2199.8

48%

4858.4

64%

Class 4

1261.9

27%

2266.4

30%

Class 5

903.2

20%

2249.0

Class 6

34.6

1%

343.0

Glacier Debris-covered

Rock Glacier

30% 5%

7540.4

TE

D

4594.7

AC CE P

Total

NU

Firn

MA

Type

RI

Percent of total (max)

SC

Water Percent Water minimum of total maximum (km3) (min) (km3)

PT

Table 7

36

ACCEPTED MANUSCRIPT Table 8

1132.2

877.7

1164.7

Class 1

190.3

223.9

Class 2

371.4

485.1

Class 3

316.0

455.7

1804.2

3914.3

Class 4

1043.9

1874.8

Class 5

740.6

1844.0

Class 6

19.7

195.5

Primeraa

Debris-covered

Rock Glacier

Total

4177.2 Firn

Class 1

D

0.0

12.3 0.0

0.0

0.0

0.0

0.0

AC CE P

Putaendob

9.7

12.3

Debris-covered

0.0

0.0

395.6

944.1

Class 4

218.0

391.5

Class 5

162.7

405.1

Class 6

14.9

147.5

417.6

966.1

Class 2 Class 3

Rock Glacier

Total

6574.3

9.7

Glacier

PT

1132.2

Glacier

RI

363.1

NU

363.1

MA

Firn

SC

Type

Water maximum (km3)

TE

Basin

Water minimum (km3)

a

Area above 2000 m = 1940 km²; above 3000 m = 1356 km²; a 30% reduction in area. b Area above 2000 m = 1073 km²; above 3000 m = 571 km²; a 47% reduction in area.

37

ACCEPTED MANUSCRIPT Table 9

31% decrease

Type Firn

0.0

0.0

Glacier Debriscovered

0.0

0.0

0.0

0.0

366.5

855.5

366.5

855.5

Rock Glacier

37% decrease

18.5

Glacier Debriscovered

13.0

136.7

176.2

Rock Glacier

662.4

1452.7

830.6

1660.4

15.7

15.7

16.1

16.1

112.6

157.9

241.3

529.1

385.7

718.8

Firn

168.8

168.8

Glacier Debriscovered

560.0

560.0

414.9

549.2

Rock Glacier

412.5

888.4

1556.2

2166.4

Firn

169.8

169.8

Glacier Debriscovered

555.4

555.4

213.5

281.4

Rock Glacier

405.8

867.9

1344.5

1874.5

Firn

0.0

0.0

Glacier Debriscovered

0.0

0.0

0.0

0.0

111.4

265.0

111.4

265.0

230

186

Rio Juncal

Total

491

377

23% decrease

Rio Blanco

Total

365

Rock Glacier

AC CE P

19% decrease

Glacier Debriscovered

D

Firn

TE

Rio Riecillos

Total

280

23% decrease

Rio Aconcagua

Total

Total

175

102

42% decrease

Rock Glacier

18.5

NU

515

Firn

MA

Rio Colorado

Total

820

Water maximum (km3)

PT

413

Water minimum (km3)

RI

601

Area above 3000 m (km²)

SC

Rio Rocín

Basin

Area above 2000 m (km²)

13.0

38

ACCEPTED MANUSCRIPT Percentage of total discharge

39.9

1.8%

2.55

18.2%

30.2 70.0 4.9 2075.0 2220.0

1.4% 3.2% 0.2% 93.5%

1.14 1.40 0.02 8.92 14.03

8.1% 10.0% 0.1% 63.6%

RI

Contribution to discharge (m3/s)

D

MA

NU

SC

Percentage of total area

TE

Glacier DebrisCovered Rock Glacier Firn Talus Total

Total area (km²)

AC CE P

Type

PT

Table 10

39

ACCEPTED MANUSCRIPT Highlights

TE

D

MA

NU

SC

RI

PT

A total of 916 cryosphere landforms were mapped in the semiarid Andes of Chile Glaciers were found at the highest elevations and on southern facing slopes Glaciers and debris-covered glaciers were the largest, but covered less total area Rock glaciers were the most abundant and accounted for 48 to 64% of water stored These landforms provide an important late season water resource

AC CE P

    

40